Further improvements to file size estimate

[sindresorhus]

#205 made the estimate much more accurate, but we can still improve it:

• We currently add 10% to ensure the estimate is always more than the actual size. The percentage could be more dynamic: Improve file size estimate #205 (comment)
• Show a range for the naive estimate: Display estimate as a range #130
• For the naive estimate:

  Once you get any better estimate, remember the ratio between the naive and the better estimate. Apply that ratio to later naive estimates. This will give you decent estimates real time for changing parameters.

• Use machine learning to improve the naive estimate?
• Refactor the code to use a different method for file size estimation:

  Gifski/Gifski/Gifski.swift

  Line 214 in e6c97bc

  // TODO: The whole estimation thing should be split out into a separate method and the things that are shared should also be split out.

Other ideas welcome.